Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 AM EDT Thu Oct 17 2019

Valid 12Z Thu Oct 17 2019 - 12Z Sun Oct 20 2019

...Pacific Northwest to the Northern Rockies...

Days 1-3...

Persistent onshore flow with embedded mid to upper level waves
will likely bring rounds of organized precipitation through today
while snow levels remain between 5000-6000ft across the northern
Cascades and Olympics.  The longer duration colder temperature
aloft lead to higher snow totals over the ranges of WA, with lower
amounts further inland.
Cold advection tonight into early Fri helps set the stage for
higher amounts over the northern Rockies Fri into this weekend. 

The next wave approaches and moves into the Pacific Northwest
Friday night into Saturday, accompanied by enhanced layer relative
humidity and 700 mb lift.  The extended periods of ascent will
deliver another round of moderate to locally heavy precipitation
as snow levels across the Olympics and northern Cascades drop to
4000-5000ft on Saturday.  Three day snowfall accumulations of
2-4ft, with locally higher amounts, are expected across the higher
peaks of the northern Cascades.  Snow levels will drop to
4500-5500ft - supporting locally heavy accumulations across the
northeastern Oregon into the central Idaho ranges on Saturday.

With the colder temperatures in place, several inches of snow are
expected Fri in the ranges of northern ID and northwest MT,
including Glacier National Park.  As a 700 mb wave moves east out
of the Pacific Northwest across MT, the bands of enhanced moisture
and lift cross the ranges of ID, western MT and northwest WY.  The
longer duration upslope flow and synoptic ascent leads to heavy
snow likely, with the highest amounts in the Tetons of northwest
WY, where the 00z ECMWF shows 300 mb divergence maxima in a
coupled jet region. QPF amounts of an inch to inch and half day 3
supports amounts near a foot/locally higher in portions of the
Bitterroots, Tetons, and OR Cascades on day 3/Sat.


...Northeast...

Day 1...
A powerful cyclone moving north into eastern New England today and
into New Brunswick Fri day night should result in low-mid level
cold advection in the western semicircle of the circulation. 
Falling heights lead to gradually falling temperatures aloft and
potential for rain to change to snow at higher elevations of the
Adirondacks, Green and White Mountains.
Several inches of snow is possible at higher elevations due to
slow movement of the upper low.  Lower elevations are forecast to
remain as mostly rain. The event ends as the low departs tomorrow.

The probability of significant icing is less than 10 percent Days
1-3.

Petersen